Pulsating B and Be stars in the Small Magellanic Cloud

TL;DR

This study investigates pulsating B and Be stars in the low-metallicity environment of the Small Magellanic Cloud, revealing the presence of beta Cephei and SPB stars and mapping their instability regions, which differ from those in the Milky Way.

Contribution

It provides the first observational evidence of B-type pulsators at the SMC's low metallicity and characterizes their properties and instability regions.

Findings

Detected 9 B stars with short-period variability, including a beta Cephei candidate.

Identified 32 Be stars as short-period variables, with 20 being multiperiodic.

Found the SPB instability strip shifted towards higher temperatures in the SMC.

Abstract

Context: Stellar pulsations in main-sequence B-type stars are driven by the kappa-mechanism due to the Fe-group opacity bump. The current models do not predict the presence of instability strips in the B spectral domain at very low metallicities. As the metallicity of the SMC is lower than Z=0.005, it constitutes a very suitable object to test these predictions. Aims: The main objective is to investigate the existence of B-type pulsators at low metallicities, searching for short-term periodic variability in absorption-line B and Be stars in the SMC. The analysis has been performed in a sample of 313 B and Be stars with fundamental astrophysical parameters accurately determined from high-resolution spectroscopy. Methods: Photometric light curves of the MACHO project have been analyzed using standard Fourier techniques and linear and non-linear least squares fitting methods. The position of the pulsating stars in the HR diagram has been used to ascertain their nature and to map the instability regions in the SMC. Results: We have detected 9 absorption-line B stars showing short-period variability, two among them being multiperiodic. One star is most likely a beta Cephei variable and the remaining 8 are SPB stars. The SPB instability strip in the SMC is shifted towards higher temperatures than the Galaxy. In the Be star sample, 32 stars are short-period variables, 20 among them multiperiodic. 4.9% of B stars and 25.3% of Be stars are pulsating stars. Conclusions: beta Cephei and SPB stars do exist at the SMC metallicity. The fractions of SPB stars and pulsating Be stars in the SMC are lower than in the Galaxy. The fraction of pulsating Be stars in the SMC is much higher than the fraction of pulsating absorption-line B stars, as in the Galaxy.